Thrombotic diseases remain a major health care problem despite the tremendous progress made in understanding the molecular mechanisms of blood coagulation and pathogenesis of thrombosis and atherosclerosis. In fact, each year in the United States, approximately 1.5 million patients experience acute myocardial infarction and 5 million patients develop angina.
Generally, thrombosis occurs from an imbalance between prothrombotic and antithrombotic mechanisms. In principle, enhanced platelet activation, increased thrombin formation and blood coagulation or reduced fibrinolytic activity all could lead to thrombosis. Currently marketed antithrombotic drugs and the majority in development are designed to inhibit platelets or blood coagulation factors. Thrombolytic agents, such as streptokinase, and recombinant tissue-type plasminogen activator (tPA) and urokinase (uPA) are used mostly for acute myocardial infarction (Verstraete, M., The fibrinolytic system: from Petri dishes to genetic engineering, Thromb Haemost 74, 25–35 (1995); 2). These protein-based drugs are designed to be administered intravenously for rapid onset of action.
Fibrinolysis is a physiological mechanism to remove intravascular thrombus and maintain vascular patency. After a blood clot is formed in an injured vessel, the fibrinolytic system degrades the fibrin clot, restoring blood flow to vital organs and tissues. The fibrinolytic system consists of several proteases, namely tPA and uPA and plasminogen, which form an enzymatic cascade in which tPA and uPA convert plasminogen to plasmin, which in turn degrades fibrin.
The fibrinolytic enzymes of the fibrinolytic system are not only physiologically important in vascular homeostasis but can also cause unwanted effects such as bleeding and excessive vascular proteolysis. Therefore, tight regulation of the fibrinolytic system is typically achieved by activation of zymogens through limited proteolysis, controlled binding of plasminogen or plasmin to fibrin, and inactivation of proteases by serine protease inhibitors, and inactivation of plasmin binding sites on fibrin by the action of plasma carboxypeptidase B. A major challenge for thrombolytic therapy is to reduce the incidence of angiographic reocclusion. In fact, angiographic reocclusion is observed in about 30% of patients three months after successful thrombolysis for acute myocardial infarction. Reocclusion significantly affects recovery of left ventricular function and leads to a poorer long-term clinical outcome. Other approaches are therefore necessary to reduce coronary reocclusion.
The first enzyme in the blood clotting cascade consists of two distinct protein subunits: a catalytic subunit, Factor VIIa, also referred to as FVIIa, and an essential regulatory subunit, tissue factor, also referred to as TF. FVIIa is a soluble serine protease, bound with its cofactor TF, a cell-surface, integral-membrane protein, responsible for the conversion of Factor X to Factor Xa. Factor VIIa is the triggering enzyme of the blood clotting cascade in hemostasis and thrombosis and may play an important role in hypercoagulable states. Indeed, certain epidemiological studies have found elevated FVII coagulant activity to be an independent risk factor for heart disease.
The tissue factor (TF) coagulation pathway is initiated when circulating FVIIa encounters TF, a cell surface glycoprotein, as a result of vascular injury or pathological perturbation. TF-induced coagulation plays a primary role in hemostasis and also in the pathogenesis of various thrombotic disorders. A Factor VIIa that is modified to be inactivated such as an active site-blocked activated factor, can be used as an antithrombotic agent based on its ability to block binding of Factor VIIa to TF.
The Factor VIIa/TF complex is involved in the pathogenic mechanism in a variety of thrombotic diseases and the circulating level of TF is a risk factor for certain patients. Factor VIIa and TF play a unique role in vascular injury in maintaining hemostasis and initiating thrombosis. TF is expressed in the adventitia normally, but is upregulated and expressed inappropriately in the media and neointima in vascular disease. TF expression in atherosclerotic plaques is increased and shielded from the blood by a thin fibrous cap that may rupture to expose TF. Surgical interventions such as balloon angioplasty, stenting, or endarterectomy damage the vessel wall and expose underlying TF. In the atherosclerotic, lipid-rich, thin-walled plaque, spontaneous rupture or endothelial erosion leads to TF exposure and thrombosis resulting in unstable angina and myocardial infarction. TF can circulate in cell-derived microparticles, and circulating TF levels are elevated in unstable angina suggesting that this circulating TF may contribute to thrombus formation. Often cancer is associated with a hypercoagulable state attributed to overexpression of TF on tumor cells. This predisposes the patient to deep vein thrombosis, pulmonary embolism and low grade disseminated intravascular coagulation (DIC). DIC results in microvascular fibrin deposition contributing to multi-organ failure. Results from acute arterial injury models of thrombosis indicate that protein-based inhibitors of Factor VIIa/TF are effective antithrombotics with less bleeding compared to thrombin and FXa inhibitors. In addition, Factor VIIa/TF inhibition is superior to other anticoagulants (heparin, FXa inhibitor) in preventing neointimal thickening and vascular stenosis following balloon injury.
However, a class of compounds is needed that are useful in treating diseases characterized by thrombotic activity. Also, compounds are needed that inhibit the activity of certain enzymes in the coagulation cascade such as Factor VIIa in vivo that overcome the problems associated with prior compounds. In addition, a class of compounds is necessary that has increased potency allowing smaller dosages than the compounds known in the prior art to prevent thrombosis and atherosclerosis, and may further be utilized to prevent cancer invasion and chemotherapy-induced fibrosis.